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Relay Circuit Schematics

Basic Setup: RPi → Relay Module → Bike Buttons

Component Overview

Raspberry Pi                   Relay Module (opto-isolated)
┌────────────┐                 ┌─────────────────────────────────┐
│            │                 │  VCC ── 5V power                │
│  GPIO 17 ──┼── IN1 ─────────┤  IN1    Relay 1 (Resistance UP) │
│  GPIO 27 ──┼── IN2 ─────────┤  IN2    Relay 2 (Resistance DN) │
│            │                 │  GND ── Ground                  │
│  5V (pin 2)┼── VCC ─────────┤                                 │
│  GND (pin 6)┼── GND ────────┤  Jumper: VCC-JD connected       │
└────────────┘                 └──────┬─────┬──────┬─────┬───────┘
                                      │     │      │     │
                                 Relay 1          Relay 2
                                 COM   N.O.       COM   N.O.
                                  │     │          │     │
                                  ▼     ▼          ▼     ▼
                               Bike + Button    Bike - Button
                               pad A  pad B     pad A  pad B

GPIO Pin Mapping (Default)

RPi GPIO Physical Pin Relay Channel Function
GPIO 17 Pin 11 IN1 Resistance UP (+)
GPIO 27 Pin 13 IN2 Resistance DOWN (-)
GPIO 22 Pin 15 - Speed sensor signal (INPUT)
5V Pin 2 VCC Relay module power
3.3V Pin 1 - Speed sensor power
GND Pin 6 GND Common ground (relay)
GND Pin 9 - Common ground (speed sensor)

RPi GPIO Header Reference (relevant pins)

  Sensor VCC ──► 3.3V [1 ] [2 ] 5V ◄── Relay VCC
                      [3 ] [4 ] 5V
                      [5 ] [6 ] GND ◄── Relay GND
                      [7 ] [8 ]
  Sensor GND ──►  GND [9 ] [10]
  GPIO17/UP  ────────► [11] [12]
  GPIO27/DN  ────────► [13] [14] GND
  GPIO22/SPEED ──────► [15] [16]
                 3.3V [17] [18]
                      [19] [20] GND
                      [21] [22]
                      [23] [24]
                  GND [25] [26]

Relay Module Wiring Detail

Understanding the relay contacts

Each relay has three terminals:

         ┌─────┐
   COM ──┤     ├── N.O. (Normally Open)
         │Relay│
   N.C.──┤     │
         └─────┘
  • COM (Common): Always connected to one side
  • N.O. (Normally Open): Disconnected when relay is off, connected to COM when relay is on
  • N.C. (Normally Closed): Connected to COM when relay is off, disconnected when on

We use COM and N.O. — the circuit is open (button not pressed) when relay is off, and closed (button pressed) when relay is on.

Wiring to bike button

Bike Button PCB
                    ┌───────────────────┐
                    │   ┌─Button──┐     │
  Pad A ──────────────┤          ├──────────── Pad B
                    │   └────────┘     │
                    │                   │
                    │   Wire from relay │
                    │   ┌─ COM         │
  Pad A ──────────────┤              ├──────── Pad B
                    │   └─ N.O.        │
                    │                   │
                    └───────────────────┘

Result: Button OR relay can close the circuit

Important: Wire in PARALLEL with the existing button, not in series. The original button should still work normally.

Enhanced Setup: With Optocoupler Isolation

If you want extra isolation (recommended for bikes with higher voltage circuits):

RPi GPIO 17 ───[330Ω]───┐
                          │ Anode
                     ┌────┴────┐
                     │ PC817   │
                     │ Opto-   │
                     │ coupler │
                     └────┬────┘
                          │ Cathode
RPi GND ─────────────────┘

                     ┌────┴────┐
                     │ PC817   │
                     │ Output  │──── Bike Pad A
                     │ side    │
                     └────┬────┘
                          │
                          └──── Bike Pad B

This removes the need for a relay module entirely if the button circuit is low-current (< 50mA). The optocoupler provides complete galvanic isolation.

Optocoupler Component Values

Component Value Purpose
R1 (LED side) 330Ω Limits current through opto LED (~10mA at 3.3V)
PC817 - Standard optocoupler, CTR > 50%

Speed Sensor (Hall Effect A3144)

Wiring

RPi 3.3V (Pin 1) ──────────────── A3144 Pin 1 (VCC)
                                       │
                                   ┌───┘
                                   │
                                [10kΩ]  ← pull-up resistor (optional if using
                                   │      RPi internal pull-up for short wires)
                                   │
RPi GPIO 22 (Pin 15) ─────────────┤
                                   │
                              A3144 Pin 3 (Signal)

RPi GND (Pin 9) ───────────────── A3144 Pin 2 (GND)

A3144 Pinout (flat face toward you, leads down)

    ┌─────────┐
    │  A3144  │  ← flat face (printed side)
    │         │
    └─┬──┬──┬┘
      │  │  │
      1  2  3

  1 = VCC (3.3V from RPi Pin 1)
  2 = GND (RPi Pin 9)
  3 = Signal → RPi GPIO 22 (Pin 15)

Mounting

  • Attach any small magnet to the outer rim of the flywheel (glue, tape, whatever holds)
  • Mount the A3144 on the bike frame, flat face pointing at the magnet path
  • Gap between sensor and magnet: 2-8mm
  • The magnet should pass the sensor once per flywheel revolution
  Flywheel rim ──────────────►  direction of rotation
  ┌───────────────────────────────────────┐
  │    ┌────────┐                         │
  │    │ Magnet │ (glued to rim)          │
  │    └────────┘                         │
  │         ↕ 2-8mm gap                   │
  │    ┌────────┐                         │
  │    │ A3144  │ (mounted to frame)      │
  │    └───┬────┘                         │
  │        │ 3 wires to RPi               │
  └────────┼──────────────────────────────┘
           ▼

Tapping Existing Bike Sensor (Alternative)

If your bike already has a speed sensor (reed switch or Hall sensor near the flywheel):

  • Reed switch (2 wires): Wire one side to GPIO 22, other to GND. Enable pull_up: true.
  • Hall sensor (3 wires): Tap the signal wire to GPIO 22 in parallel (don't disconnect from bike's circuit).
  • Caution: If the bike sensor runs at 5V, use a voltage divider (10kΩ + 20kΩ) to bring the signal to 3.3V-safe levels.

Power Considerations

Relay module power

  • Most 2-channel relay modules draw 70-100mA total
  • RPi 5V pin can supply up to ~300mA safely
  • If using more relays, consider a separate 5V power supply

Speed sensor power

  • A3144 draws < 10mA at 3.3V — negligible
  • Powered from RPi 3.3V pin (Pin 1) which can supply up to ~50mA

GPIO current

  • RPi GPIO pins can source ~16mA each (max)
  • Opto-isolated relay modules typically draw 5-10mA per input — well within limits
  • Speed sensor signal pin draws effectively 0mA (high-impedance input)

Wiring Checklist

Relay module

  • RPi powered off during wiring
  • Bike unplugged from mains during wiring
  • Relay module VCC connected to RPi 5V (Pin 2)
  • Relay module GND connected to RPi GND (Pin 6)
  • GPIO 17 (Pin 11) connected to relay IN1 (resistance up)
  • GPIO 27 (Pin 13) connected to relay IN2 (resistance down)
  • Relay 1 COM connected to bike + button pad A
  • Relay 1 N.O. connected to bike + button pad B
  • Relay 2 COM connected to bike - button pad A
  • Relay 2 N.O. connected to bike - button pad B
  • All connections secure (soldered or firmly crimped)
  • No bare wire touching other components
  • Original buttons still function when relay is off
  • Test with python scripts/test_buttons.py before connecting to bike

Speed sensor

  • A3144 VCC (pin 1) connected to RPi 3.3V (Pin 1)
  • A3144 GND (pin 2) connected to RPi GND (Pin 9)
  • A3144 Signal (pin 3) connected to RPi GPIO 22 (Pin 15)
  • 10kΩ pull-up resistor between 3.3V and signal (optional for short wires)
  • Magnet securely glued to flywheel rim
  • Sensor mounted on frame, 2-8mm gap from magnet path
  • Magnet clears sensor without contact when flywheel spins
  • Test with python scripts/test_speed_sensor.py — spin flywheel by hand